The present invention relates generally to a double-passage acousto-optical device, that is, an acousto-optical device through which light passes twice, and specifically to various optical filters, wavelength add/drop devices, and lasers that are constructed using the double-passage optical device.
It is known to use optical fibers to send optical information-carrying signals for long-distance communication.
Optical telecommunication systems are known that use wavelength division multiplexing (WDM) transmission. In these systems several channels, i.e. a number of independent transmission signals, are sent over the same line by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
The Applicant has observed that known WDM communication systems are limited as concerns the number of channels, i.e. the independent wavelengths that can be used for transmission within the wavelength band available for signal transmission and amplification.
In order to combine and separate signals with different wavelengthsxe2x80x94to combine the signals at the transmission station, for example, to drop some toward receivers located at intermediate nodes of the line or to introduce others at intermediate nodes or to send them to separate receivers at the receiving stationxe2x80x94adjacent channels (in wavelength terms) must be separated by more than a minimum predetermined value.
Said minimum value depends on the characteristics of the components employed in the system, such as the spectral characteristics of the wavelength selective components (e.g. bandwidth, center-band attenuation, figure of merit) and the wavelength stability (thermal and temporal) of the selective components themselves and of optical signal sources.
In particular, the Applicant has observed that the spectral selectivity of currently available wavelength selective components may greatly limit the possibility of adding and dropping signals in multichannel transmission systems, particularly when there are signals with closely spaced wavelengths, e.g. separated by less than 2 nm.
In optical filtering, either in optical communication systems or for other purposes, double-stage optical filters are advantageous, because their filtering performance is increased compared to a single-stage filter having the same characteristics. Acousto-optical filters are known that provide for the interaction between light signals, propagated in waveguides formed on a substrate of birefringent and photoelastic material, and sound waves propagated on the surface of the substrate. The sound waves are generated by suitable transducers and are initially supplied by radio frequency signals.
The interaction between a polarized optical signal and a sound wave produces a polarization conversion of the signal, in other words, a change of the polarization from its transverse electric TE component to its transverse magnetic TM component, which are orthogonal to each other, and vice versa. Following this interaction with the sound wave, the polarization components undergo not only the conversion to the corresponding orthogonal components, but also a frequency shift whose absolute value is equal to the frequency of the interacting sound wave (and therefore equal to that of the applied radio frequency signal). The sign of the frequency shift is a function of the state of polarization and of the direction of propagation of the sound wave with respect to the optical wave.
In addition to their use as pure filters, acousto-optic devices have also been used as tuning devices for lasers. To Applicants"" knowledge, however, conventional acousto-optic devices used as filters or tuners for lasers have generally employed either a single-stage acousto-optic substrate using a single pass of light through the device or have employed multiple stages of cascaded acousto-optic filters that are constructed on separate physical substrates.
For example, EP Application 97113188.3 describes an acousto-optic device including a substrate of a material capable of propagating a surface acoustic wave along a portion of the surface of the substrate, a transducer for generating the surface acoustic wave, an optical waveguide formed in a substrate, and an acoustic absorber surrounding the portion of the substrate.
EP 0814364 A1 describes a double-stage acousto-optical waveguide device. FIG. 12 shows a switch, or add/drop node comprising, in addition to a third polarization conversion stage 303, a fourth polarization conversion stage 403. The fourth polarization conversion stage 403 is connected to an input polarization splitter 404 and to an output polarization splitter 405. In turn the splitter 405 is connected to the polarization splitter 204 by means of the connecting branch 210 and to the lateral waveguard 255. The ports 19, 20, 21 and 22 are connected to the line. The polarization splitter 404 is connected to input ports 25 and 26 through which the signals to be added or subtracted are introduced and signals to be added or subtracted are also introduced through the ports 23 and 24.
U.S. Pat. No. 5,611,004 discloses a polarization independent acousto-optical tunable filter (AOTF). The patent describes in its FIG. 6 an embodiment where two stages of signal filtering are realized with only one transducer 43 on the substrate 31. Two stages of filtering are realized by passing the incoming beam of light through the AOTF a first time, reflecting the beam of light off of a mirror 67 and then passing the beam of light through the same AOTF a second time. A band pass filtered representation of the original beam of light is obtained at a circulator output 71 of an optical circulator 69 located at the input of the embodiment.
U.S. Pat. No. 5,452,314 describes an acousto-optic tunable filter with a pair of electrodes on opposite sides of the waveguide. The patent discloses the use of a voltage source in which an applied electric field controls the birefringence of the filter, and a tunable laser incorporating such an acousto-optic tunable filter. Suitably adjusting the potential applied by the voltage source results in suppression of sidelobes, correction of asymmetric sidelobes, and compensation for physical variations in the waveguide.
U.S. Pat. No. 5,002,349 and EP 805372 describe single converter acousto-optic tunable filters. The ""349 patent discloses an acousto-optical converter that allows multiple stages of such converters so as to provide for two-stage zero-frequency shifted converters and filters, lasers using an acousto-optic filter as a tuning element, polarization-independent converters, and wavelength-division-multiplexing routing switches. xe2x80x9cAcoustically tuned erbium-doped fiber ring laserxe2x80x9d by D. A. Smith et al., Optics Letters, vol. 16, no. 6, Mar. 15, 1991, describes a continuously tunable laser that uses an acousto-optic filter to achieve a broad tuning range. xe2x80x9cAcoustically tunable Ti:Er:LiNbO-Waveguide Laserxe2x80x9d by I. Baumann et al., ECOC ""94, vol. 4, pp. 99-102 describes a waveguide laser comprising a double-stage wavelength filter utilized as a narrow-band tunable optical amplifier.
WO 98/11463 discloses, with reference to its FIG. 7, an embodiment that can alleviate the polarization sensitivity of a null coupler acousto-optic tunable filter by simultaneously applying two acoustic waves. The null coupler is made from two optical fibers with diameters that are mismatched to the extent that the resultant coupler gives an extremely small passive coupling efficiency. The coupler is made by pre-tapering one of the two identical single-mode fibers along a short length before both fibers are fused and elongated together to form the coupler.
In the device of WO 98/11463, input light in the fiber that was not pretapered excites only the fundamental mode in the narrow waist of the coupler. Light in the other fiber excites only the second order mode. When the acoustic wavelength matches the optical beat length of the two modes in the waist, resonant coupling takes place between them. After light enters the device at port 1, both polarization components are coupled to port 2 but undergo different upshifts in frequency. After propagating through a loop of a polarization independent isolator, light of each polarization re-enters the coupler and is coupled a second time, exiting through port 4 with a frequency downshift.
In contrast, devices that use polarization converters as opposed to modal converters as in WO 98/11463 use two separate waveguides with birefringent material rather than fused fiber couplers. In the fiber coupler arrangement, the optical signal passes through only a single waveguide and conversion region. The modal coupling device in WO 98/11463 also faces a high polarization sensitivity that requires two separate radio frequencies to overcome.
Applicants have discovered that double-stage acousto-optic filters having polarization converters as opposed to modal converters provide a lower than desired level of filtering for optical signals due to the physical layout of the configuration. In particular, Applicants have observed that in conventional double-stage optical filters, it may be difficult to match the filter characteristics of the two stages, particularly when the stages are located physically apart. Similarly, Applicants have discovered that matching the temperatures of the two stages may present difficulties as well. Further, two separate stages may require separate driver frequencies for their respective acousto-optical devices, which may be mismatched, and therefore must be separately calibrated.
Applicants have noticed that the amount of filtering attainable with an acousto-optic filter depends significantly on the configuration chosen for the filter and its affiliated components. Particularly, Applicants have identified that a single pass and single stage acousto-optic filter provides insufficient filtering performance. As well, a two-stage filter integrated on a single substrate introduces numerous manufacturing challenges and risks in properly balancing the temperature and stability between the two stages.
Applicants have, therefore, developed an inventive acousto-optic filter and laser using the filter for double-pass and single stage operation. The filter of the present invention enables single temperature control (no temperature difference between two stages), a single radio frequency control, reduction to half of the electro-acoustic transducers and of the radio-frequency power with respect to a double-stage device, less overall size, narrower bandwidth, and suitability for use as a filter with notch output and as an add/drop device. As well, this filter provides the capability of eliminating frequency shift and wavelength shift, if necessary, and performing an effective polarization independent device, and the capability of eliminating frequency shift even when the radio frequency on the two converters is different.
In one aspect, a double-passage acousto-optical device includes an acousto-optical switch including first and second polarization conversion regions both coupled between first, second, third, and fourth optical ports, together with an optical combination coupled between the second and third optical ports that includes an optical isolating element. Preferably, a first optical splitter is positioned between the first and fourth optical ports and the first and second polarization conversion regions, where the first optical splitter has cross and bar transmission respectively for orthogonal polarization components of received light. Also, a second optical splitter is preferably positioned between the second and third optical ports and the first and second polarization conversion regions, where the second optical splitter has cross and bar transmission respectively for orthogonal polarization components of received light.
More preferably, the double-passage acousto-optical device consistent with the present invention includes an upper transducer within the acousto-optical switch acoustically coupled to the first polarization conversion region and coupled to an RF source, where the upper transducer generates a first acoustic wave in the first polarization conversion region that has a characteristic frequency determined by the RF source. As well, the double-passage acousto-optical device further includes a lower transducer within the acousto-optical switch acoustically coupled to the second polarization conversion region and coupled to the RF source, where the lower transducer generates a second acoustic wave in the second polarization conversion region that has the characteristic frequency with a propagation direction opposite to a propagation direction of the first acoustic wave.
In another aspect, a tunable laser generator that uses an acousto-optical device includes an acousto-optical switch having first and second polarization converters coupled between first, second, third, and fourth optical ports, a first optical half-ring coupled between the first and fourth optical ports and including an optical amplifier, and a second optical half-ring coupled between the second and third optical ports and including an optical isolating element. The laser includes a laser output coupler positioned within the first optical half-ring. Preferably, the tunable laser generator has an optical isolating element in the first half-ring. Preferably, one or both of the first half-ring and the second half-ring consist of polarization-maintaining elements. Alternatively, one or both of the first half-ring and the second half-ring includes a polarization controller. Preferably an RF generator is coupled to the first and second polarization converters for tuning the laser generator.
Preferably, the tunable laser generator further comprises a first optical splitter positioned between the first and fourth optical ports and the first and second polarization converters, the first optical splitter having cross and bar transmission respectively for orthogonal polarization components of received light. Advantageously it further comprises a second optical splitter positioned between the second and third optical ports and the first and second polarization converters, the second optical splitter having cross and bar transmission respectively for orthogonal polarization components of received light.
Preferably, the tunable laser generator further comprises an upper transducer within the acousto-optical switch acoustically coupled to the first polarization converter and coupled to an RF source, the upper transducer generating a first acoustic wave in the first polarization converter and having a characteristic frequency determined by the RF source. In an embodiment, it further comprises a lower transducer within the acousto-optical switch acoustically coupled to the second polarization converter and coupled to the RF source, the lower transducer generating a second acoustic wave in the second polarization converter and having the characteristic frequency with a propagation direction opposite to a propagation direction of the first acoustic wave. According to a different, alternative, embodiment the first and second polarization converters are positioned in sufficient proximity that the first acoustic wave travels in both the first and second polarization converters.
In another aspect of the present invention, a method of filtering an optical frequency using an acousto-optical device includes the steps of providing at least one acousto-optical switch including first and second polarization conversion regions coupled between first, second, third, and fourth optical ports; injecting an input optical signal having a plurality of wavelengths into the first optical port; feeding-back an intermediate optical signal including a subset of the plurality of wavelengths from the second optical port to the third optical port via an optical isolating element; and extracting an output optical signal including the subset of the plurality of wavelengths from the fourth optical port. Preferably, the method further includes the step of extracting a remainder of the plurality of wavelengths from the second optical port via the optical isolating element, wherein the optical isolating element is an optical circulator having at least three ports. Alternatively, the method further includes the step of inputting additional wavelengths to the device via the optical isolating element. Alternatively, the method can further comprise the step of controlling the polarization of the intermediate optical signal, or the step of maintaining the polarization of the intermediate optical signal using polarization-maintaining components.
More preferably, the method consistent with the present invention includes the steps of splitting the input optical signal into TE and TM initial components within a first optical splitter on the switch; passing the TE initial component to the first polarization conversion region; passing the TM initial component to the second polarization conversion region; orthogonally converting the TE initial component of the input optical signal having a selected frequency to a TM intermediate component in the first polarization conversion region; and orthogonally converting the TM initial component of the input optical signal having the selected frequency to a TE intermediate component in the second polarization conversion region. Alternatively, the method further includes the step of combining the TE and TM intermediate components into the intermediate optical signal or combining the TE and TM initial components into the intermediate optical signal.
Also preferably, the method includes, after the feeding-back step, the steps of splitting the intermediate optical signal into TE and TM feedback components within a second optical splitter on the switch; passing the TE feedback component to the first polarization conversion region; passing the TM feedback component to the second polarization conversion region; orthogonally converting the TE feedback component having the selected frequency to a TM final component in the first polarization conversion region; and orthogonally converting the TM feedback component having the selected frequency to a TE final component in the second polarization conversion region. Alternatively, the method further includes the step of combining the TE and TM final components into the output optical signal or combining the TE and TM feedback components into the output optical signal.
Applicants note that in the double-passage acousto-optical device according to the present invention, the filter characteristics are always matched and aligned; there is no need for temperature matching; and only a single driver frequency is needed for the acousto-optical filter therein.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of this invention.